This fifth paper in the soft tissue series deals with common injuries to the knee, together with assessment, imaging and therapy considerations.

The structures within the knee joint are frequently injured, particularly in sport, and assessment can be difficult for the non-specialist. Patients presenting with injuries to the knee should be approached in the same way as patients presenting with musculoskeletal problems elsewhere—a history should be taken, then you should look, feel, move (actively and passively), strain to assess ligament integrity and image if indicated.

ANATOMY AND BIOMECHANICS

Although the knee might appear to be a simple hinge it is in fact a complex joint, allowing both sliding and rotation of the femur on the tibia in addition to simple flexion and extension.1 As extension progresses, the shorter, more highly curved lateral condyle comes to the end of its articular surface and its movement is checked by the anterior cruciate ligament (ACL). Meanwhile, the larger and less curved medial condyle continues its movement, consequently sliding backward, assisted by tightening of the posterior cruciate ligament (PCL). The result is medial rotation of the femur (or external rotation of the tibia), which serves to tighten the collateral ligaments and lock the joint. In addition, the joint moves beyond full extension by approximately 5° helping to lock it in an extended position (see fig 1).

As the knee flexes from full extension, movement is preceded by lateral rotation of the femur (or medial rotation of the tibia), assisted by the popliteus muscle. Rotation relaxes the tension of the collateral ligaments and flexion then occurs.

In fig 2 the knee is open to display more clearly the ligaments and the menisci. The medial collateral ligament limits abnormal valgus movement of the joint and the lateral collateral limits abnormal varus movement (fig 3).

The cruciate …